1. Field of the Invention
The present invention relates to an optical scanner that can be used in an image forming apparatus such as a digital copier, a laser printer, a laser facsimile and so forth, and to an image forming apparatus that includes an optical scanner.
2. Description of the Related Art
An optical scanner is used in laser printers or the like to scan a scanning surface and obtain image data of the scanning surface. A typical optical scanner includes a light deflector that receives a light beam from a light source and deflects the light beam, and a scanning/imaging optical system such as a fθ lens that condenses the deflected light beam to create a beam spot on a scanning surface and scan the scanning surface with the beam spot. The direction in which the beam spot scans the scanning surface is called a main scanning direction and the process is called main scanning. In practice, a photo-sensitive surface of a photoconductive drum or the like is a typical example of the scanning surface.
Full-color image forming apparatuses include a plurality of photoconductors, e.g., four, aligned along a conveying direction of recording paper; a deflector that deflection-scans the light beam of light beams emitted by a plurality of light source units, corresponding respectively to the photoconductors; a plurality of scanning/imaging optical systems, corresponding respectively to the photoconductors that simultaneously expose the photoconductors thus to create a latent image; and a developer that employs developing agents of different colors such as, for example, yellow, magenta, cyan and black, to thereby visualize the latent image, so that a colored image is obtained upon sequentially transferring the visual images onto a recording paper and fixing the total image. Such an image forming apparatus that includes two or more sets of optical scanners and photoconductors to produce two-color, multi-color or full-color images is known as a tandem-engine image forming apparatus. Some tandem-engine image forming apparatuses include a plurality of photoconductive media and a single deflector for shared use. Examples of such image forming apparatuses are given below.
(1) An apparatus that has a plurality of light beams generally parallel to one another and separated in a sub-scanning direction made incident to a light deflector for deflecting the respective deflected light beams through a plurality of scanning optical elements aligned in the sub-scanning direction so as to, respectively, correspond to the light beams, so that the light beams are scanned on a surface of photoconductors, respectively, corresponding to each light beam is disclosed in Japanese Published Unexamined Patent Application No. H09-54263.
Accordingly, a separate scanning optical system is independently provided so as to handle each light beam.
(2) An apparatus provided with a scanning optical system including three optical elements L1, L2 and L3, wherein a plurality of light beams are made incident upon one side of a light deflector, so that the light beams directed to different scanning surfaces are transmitted through two of the optical elements L1, L2, and the remaining optical element L3 is disposed on each scanning surface, is disclosed in Japanese Published Unexamined Patent Application Nos. 2001-4948, 2001-10107 and 2001-33720. In this apparatus, a part of the optical elements constituting the scanning optical system is shared by the light beams, and the rest of the optical elements are disposed so as to respond to each light beam, respectively.
These apparatuses employ a single light deflector to be shared by a plurality of scanning surfaces, which allows a reduction in the number of light deflectors, thus reducing the dimensions of the image forming apparatus. This also applies to an optical scanner of a full-color image forming apparatus that includes scanning surfaces (photoconductors) compatible with four different colors, such as, cyan, magenta, yellow and black. When used in the optical scanner of such a full-color image forming apparatus, however, the light deflector (for example a polygon mirror), has to be sufficiently large in the sub-scanning direction. This is because the light beams directed in the sub-scanning direction to the photoconductors are aligned generally parallel to each other when entering the light deflector. In general, the cost of the polygon mirror (light deflector) section comprises a considerable portion in the cost of optical elements forming the optical scanner. Therefore, employing a large polygon mirror limits the desired reduction in cost and dimensions of the overall imaging apparatus.
Accordingly, an oblique incidence optical system, in which the light beam is made incident upon the deflecting surface of the light deflector with an angle in the sub-scanning direction has been developed, in an attempt to reduce the cost of the optical scanner of a color image forming apparatus by employing a single light deflector, as disclosed in Japanese Published Unexamined Patent Application No. 2003-5114. In the oblique incidence optical system, the light beams are separated by a beam bending mirror or the like after being reflected by the deflecting surface, thus the light beams are led to the corresponding scanning surface (photoconductor). Here, the angles of the respective light beams in the sub-scanning direction (the oblique incidence angle to the light deflector) are determined so as to allow the beam bending mirror to separate the light beams. Adopting such an oblique incidence optical system allows for the necessary spacing between the light beams in the sub-scanning direction (so as to enable the beam bending mirror to separate the light beams), without increasing the size of the light deflector, i.e., without increasing the number of elements or the thickness of the polygon mirror in the sub-scanning direction.
There are a number of issues when employing a polygon mirror as the light deflector in the oblique incidence optical system. It is difficult to allow the light beam from the light source enter the rotation axis of the polygon mirror, by an ordinary incidence system. When the light beam is made incident upon the rotation axis of the polygon mirror, each deflecting surface inevitably must be very large in order to secure a necessary deflection angle, which inhibits reducing the dimensions of the polygon mirror. A larger deflecting surface incurs a greater “sag.” The sag thus generated is asymmetric with respect to the field height: 0, which makes subsequent corrections more complicated. Moreover, a larger polygon mirror requires larger energy for high-speed rotation thereof, and since wind noise due to the high-speed rotation naturally becomes larger, a larger scale sound shielding means is also required.
Using, the oblique incidence system, however, allows the light beam from the light source to be made incident on the rotational axis of the polygon mirror. Therefore, the polygon mirror can be made in smaller dimensions, thereby reducing the wind noise from the high-speed rotation. This makes the oblique incidence system appropriate for high-speed operation. Reducing the size of the polygon mirror will naturally reduce the sag, in which case the sag can be made symmetric with respect to the field height: 0 and subsequent corrections can then be easily made.
The oblique incidence system has, however, a drawback that a larger scanning line curvature is incurred. The amount of curvature in the scanning line differs depending on the oblique incidence angle in the sub-scanning direction with respect to the deflecting surface of each light beam. This difference is visualized upon developing the latent image drawn by each light beam on the photoconductor with a toner of the corresponding color, resulting in color deviation when the toner images are stacked. Moreover, because of the oblique incidence angle, the light beam is twisted when entering the scanning lens, which increases the wavefront aberration, thereby significantly degrading the optical performance, especially at peripheral field height and thus increasing the beam spot diameter. This increase in wavefront aberration and beam spot diameter inhibits achieving a higher image quality.
The increased scanning line curvature is caused because in the oblique incidence system, the light source must be disposed at a position overlapping the optical axis of the scanning lens in the sub-scanning direction, so as to direct the light beam from the light source to the rotation axis of the polygon mirror. Because of this configuration, the oblique incidence angle to the deflecting surface must be increased in order to avoid interference with the scanning lens. This increased surface leads to the increase in the scanning line curvature and the problems thereby incurred, which are previously discussed.
In order to correct for the large scanning line curvature unique to the oblique incidence system, for example, Japanese Published Unexamined Patent Application No. H11-14932 proposes employing, in the scanning/imaging optical system, a lens having the specific surface inclination in the sub-scanning cross-section modified in the main scanning direction so as to correct the scanning line curvature. Japanese Published Unexamined Patent Application No. H11-38348 proposes including, in the scanning/imaging optical system, a correcting reflection surface having the specific surface inclination in the sub-scanning cross-section modified in the main scanning direction so as to correct the scanning line curvature.
Also, Japanese Published Unexamined Patent Application No. 2004-70109 proposes transmitting the obliquely incoming beam through a position deviated from the axis of the scanning lens and employing a surface that varies the amount of aspherical portion in the child line of the scanning lens along the main scanning direction, to thereby correctly align the scanning lines. However, in the invention according to Japanese Published Unexamined Patent Application No. 2004-70109, in which the correction is performed with a single lens, no reference is made about the degradation in beam spot diameter due to the increase in wavefront aberration described below, though the scanning line curvature can be corrected.
Another drawback of the oblique incidence system is that significant degradation due to wavefront aberration is prone to be incurred in the peripheral field height, because of a light skew. The wavefront aberration leads to an increase in beam spot diameter in the peripheral field height. Solving this problem is indispensable in satisfying the recent increasing demand for higher density optical scanning. Although the optical scanner according to Japanese Published Unexamined Patent Application No. 2004-70109 effectively corrects for the large scanning line curvature unique to the oblique incidence system, this invention does not provide an effective solution for wavefront aberration.
Japanese Published Unexamined Patent Application No. H10-73778 proposes an optical scanner that includes a plurality of rotating asymmetrical lenses in the scanning/imaging optical system, such that the profile of the generating line connecting the vertices of the rotating asymmetrical lens surfaces is curved in the sub-scanning direction, for effectively correcting the scanning line curvature and the wavefront aberration incidental to the oblique incidence system.
However, the lenses having “surfaces that form the generating line connecting the vertices thereof curved in the sub-scanning direction” solve the problems associated with the oblique incidence system by curving the generating line. This method requires scanning lenses to be individually provided for each incoming beam. Accordingly, employing such lenses in the tandem-engine scanning optical system requires a corresponding number of scanning lenses to the number of light beams. When a plurality of light beams directed to different scanning surfaces are made incident upon a single lens, curving the profile of the generating line can solve the problems with respect to one of the light beams, however, it is difficult to reduce the scanning line curvature or wavefront aberration with respect to the other light beam.
Moreover, the optical scanner according to Japanese Published Unexamined Patent Application No. H10-73778 has a drawback that, because of the curvature in the sub-scanning direction, when the light beams incident upon the scanning lens are shifted in the sub-scanning direction (e.g., by an assembly error, processing error or environmental fluctuation), the profile of the scanning line curvature is varied by an influence of the refracting power of the lens in the sub-scanning direction. This variation results in a failure of achieving the initial or specified performance of color difference prevention, and hence results in color deviation in the printed color image.
Further, in the aspect of correction of wavefront aberration, since the shift of the incoming beam creates a significant fluctuation of light beam skew for a surface with a curvature, it is difficult to stably obtain the appropriate beam spot diameter.
The invention according to Japanese Published Unexamined Patent Application No. 2003-5114, which proposes an oblique incidence system, also employs a similar surface to that of Japanese Published Unexamined Patent Application No. H10-73778 to thereby correct for the scanning line curvature. However, for the reasons described so far, the appropriate beam spot diameter cannot be stably obtained by the invention of Japanese Published Unexamined Patent Application No. 2003-5114.